i have a 87 s10 t5 in my 52 f3. it works great but the speedometer is ten mph fast. i know that the t5s have a choice of three different (colors) of speedometer gears located on the output shaft. im wondering which color i need. if i remember right the one in there is green. the overdrive i think is .072. tires are 235 75r 15s

 

I'm not sure you get a choice of the driving gear, the one on the tailshaft. I believe you have several choices for the driven gear that goes on the end of the cable. The driving gear on a Mustang 4-cyl T5 I junked out (which I assume had much smaller tires and a rear axle in the mid-3's) was yellow. If you search the Mustang forums, I'm sure someone has a calculator for which gear you need based on tires, rear axle ratio, etc. but it will assume a modern speedo, which may be different than an old Ford speedo.

For about $50 (I think) there is a gearbox that fits on the cable and lets you change ratios externally, over a wide range. They are on eBay

 

I don't know anything about colors, but if you can find a driven gear with 2 teeth more than the one you have currently, you should be pretty close.

 

I meant to ask, your speedo reads 10 mph fast at what "real" speed?